1. Field of the Invention
This invention relates to a technique of processing a video signal in a plasma display panel, and more particularly to a method and apparatus for processing a video signal in a plasma display panel that is capable of eliminating pseudo contour noise without any loss of gray scale level.
2. Description of the Related Art
Recently, a plasma display panel (PDP) has been highlighted as a display device having thin thickness and light weight. The PDP displays a picture by changing a light-emission frequency in proportion to a video signal such as a television signal, etc. More specifically, such a video signal is digitized, and this digitized video data is divided into sub-field periods in accordance with the number of bits. In each sub-field period, a light-emission having a frequency proportional to a weighting value of the digital video data is conducted to provide a gray scale display.
For instance, when it is intended to display a picture of 256 gray levels using 8-bit video data, one frame display period equal to {fraction (1/60)} second (i.e. 16.67 msec) is divided into 8 sub-field periods SF1 to SF8 as shown in FIG. 1. Each sub-field period SF1 to SF8 again is divided into a reset period RP, an address period AP and a sustain period SP. Herein, the reset period RP and the address period AP are equally assigned for each sub-field period. The sustain period SP is increased at a ratio of 2n (wherein n=0, 1, 2, 3, 4, 5, 6 and 7) at each sub-field.
The PDP driven with such a sub-field system superposes a light emitted in each sub-field period to display a picture corresponding to a gray level value. However, a picture display is made by superposing a light generated in the sub-field period, pseudo contour noise is generated due to inconsistency between an integration direction of light and a visual characteristic sensed by a human eye. Herein, the pseudo contour noise is generally observed a white band shape or a black band shape in which gray levels having a large difference in an light-emitting pattern, such as 127-128, 63-64 or 31-32, etc., are successively displayed.
When a light-emitting pattern is changed into 128-127, a brightness difference between two frames has a value of “1”. However, as shown in FIG. 1, the first to seventh sub-field SF1 to SF7 is emitted when a gray level value of 127 is displayed; while the eighth sub-field SF8 is emitted when a gray level value of 128 is displayed. In other words, if an emitting pattern is changed into 128-127, then a time difference in emitting patterns between two frames becomes large to cause a large movement of emitting point.
FIG. 2 depicts an amount of pseudo contour noise sensed by a human's retina. Furthermore, FIG. 2 represents a gray level value of a picture displayed at the retina when images of 127 and 128 are shifted, three by three, to the left. In FIG. 2, W0 to W6 represent real positions at which a picture is sensed from the retina.
Referring to FIG. 2, a light-emission corresponding to each bit makes an affect to a position sensed by the retina as well as the next position sensed by the retina. In other words, a gray level value of 127 emerging at W0 position is made by a summation of a gray level value of 127 having been emitted from the previous pixel and a gray level value of 127 emitted from the current pixel.
As mentioned above, since a light-emission generated from any one of pixels makes an affect to adjacent pixels, gray level values sensed from W3, W4 and W5 positions, which are boundary portions between 127 and 128, are dramatically reduced to 55.67, 29.33 and 114.3, respectively, when a gray level value is changed from 127 into 128. In other words, at the W3 position, a gray level value of 55.67 is displayed due to an influence caused by a portion of a gray level value of 127 having been emitted from the previous pixel. At the W4 position, gray level values of 127 and 128 are positioned at the boundary portion to display a gray level value of 28.33. At the W5 position, a gray level value of 114.3 emerges from a gray level value of 128. As a result, pseudo contour noise taking a black stripe shape is generated from a boundary portion between the 127 frame and the 128 frame.
In order to eliminate such a pseudo contour noise, there has been suggested an error diffusion method, an equalizing pulse method and a method of changing a sequence of sub-fields, etc. In these methods, the equalizing pulse method has been known as the best scheme for reducing pseudo contour noise without any increase of sub-field. The equalizing pulse method is a scheme of increasing or decreasing a data at a position where pseudo contour noise is generated, as shown in FIG. 3, to display a picture close to an initial data.
More specifically, in FIG. 2, a gray level value of 127 is displayed at the W3 position where a gray level of 55.67 has been expressed. When a gray level of 127 is expressed at the W3 position, a gray level value of 128 emerges from the W3 position. Further, a gray level value of 44 is displayed at the W4 position where a gray level value of 29.33 has been expressed. In this case, a gray level value of 128 emerges from the W4 position by a summation of a portion of a gray level value having been displayed at the W3 position and a gray level value of 44. Likewise, a gray level of 13 is displayed at the W6 position where a gray level value of 114.3 has been displayed to express a gray level value at the W6 position.
In other words, the equalizing pulse method can change a data applied to the sub-field to display a picture close to an initial data. Accordingly, the equalizing pulse method has an advantage in that it can eliminate pseudo contour noise from ten or less sub-fields.
In order to apply the equalizing pulse method to a PDP, a knowledge about an affect of a data motion made to the current pixel and the pixels adjacent to the current pixel is needed.
TABLE 1W0W1W2W3B00.821170.17883B11.2276160.722383B21.8184762.181524B32.1272415.872757B41.08370714.91629B527.220954.779047B638.1009125.8991B720.1182243.88177B82.223761.688130.088168SUM7.12821111.5356136.2480.088168
The above Table 1 indicates weighting values making an influence to adjacent pixels by each bit emission when an image of the PDP is shifted, three pixel by three pixel, to the left and the sustain period SP is set to a ratio of 1:2:4:8:16:32:64:64:64.
In Table 1, W0 to W3 represent positions of the retina. Herein, W1 represents a gray level displayed at the current pixel in correspondence with a data. W0, W2 and W3 represent gray levels making an affect to pixels adjacent to W1.
FIG. 4 illustrates an amount of pseudo contour noise sensed by a human's retina that is calculated with the aid of the above table 1. Further, FIG. 4 indicates a gray level value of a picture displayed at the retina when images of 127 and 128 are shifted, three picture by three picture, to the left. In FIG. 4, W0 to W6 represent positions where a picture is sensed by the retina.
Referring to FIG. 4, when a picture is moved, a gray level of 119.8718 emerges from the W2 position. Further, a gray level of 53.02007 emerges from the W3 position. Accordingly, pseudo contour noise is generated from the W3 position. The equalizing pulse method adds a gray level to a portion at which a gray level is insufficient like W2 and W3, thereby displaying a desired picture. For instance, in the equalizing pulse method, an additional gray level is applied to a portion at which a gray level is insufficient as shown in FIG. 5.
Referring to FIG. 5, a gray level of 63 is added to a gray level of 127 that is positioned at the boundary portion between 127 and 128 gray levels. Further, a gray level of 15 is added to a gray level of 128 that is positioned at the boundary portion between 127 and 128 gray levels.
If a gray level is additionally applied as mentioned above, then a gray level of 128.14 emerges from the W2 position while a gray level of 123.6637 emerges from the W3 position. In other words, in the equalizing pulse method, an additional gray level is applied to a portion at which a gray level is insufficient, thereby displaying a picture in which pseudo contour noise is reduced.
However, such a conventional equalizing pulse method cannot implement an exact image according to a data. In other words, a gray level of 128 must be displayed at the W3 position, but a gray level of approximately 124 is displayed at the W3 position. In real, the conventional equalizing method can display a picture having considerably reduced pseudo contour noise, but it has a problem in that a large difference is generated between a gray level at a position to which an equalizing pulse is applied and a real gray level to be displayed.